Currently, various communication standards such as Bluetooth, ZigBee, Wi-Fi, Wi-Fi direct, etc. as well as a cellular communication function are being applied to various digital devices. That is, communication functions are being added to most digital devices due to user requirements to freely exchange data anytime and anywhere according to service kinds regardless of coverage/device kind.
Another user requirement is high-rate data transmission. Traditionally, data transfer rate was a most the important performance requirement in wired/wireless communication and various attempts to improve data transfer rate have been conducted. In particular, in a wireless environment, environmental conditions inferior to a wired environment, such as user movement speed, presence of obstacles, channel environment related to distribution of obstacles, path loss according to distance, resource assignment in a time/frequency domain, etc., must be considered.
As one method for compensating for long-term fading such as path loss or shadowing, it is to minimize a distance between a source device and a destination device. In general, the relationship between path loss and distance is expressed in the form of an exponential function. In addition, as distance between source device and destination device increases, probability that an obstacle is present between the source device and the destination device increases.
As a method of solving short-term fading due to Doppler spread according to mobility and multipath delay spread, a multi-input multi-output (MIMO) scheme may be used. In this scheme, a scheme of using a plurality of antennas in a transmission device and/or a reception device to obtain beam gain using an antenna array, a transmit diversity scheme of simultaneously transmitting a single data stream via multiple channels to improve data transmission and reception reliability, a spatial diversity scheme of transmitting multiple data streams via different channels to increase channel capacity, etc. may be used. Although the MIMO scheme was conventionally applied to only Wi-Fi, cellular communication, etc., the MIMO technology is now expanding to new applications.
However, in the above-described schemes and functions, the MIMO scheme is not applied to most low-power wireless communication systems or a transmission device and a reception device must know mutual antenna configuration information in even a communication system to which the MIMO scheme is applied. This is because the MIMO scheme is generally applied given known configuration information including the number of antennas of the transmission device and the reception device (or in a state in which antenna configuration information can be confirmed through predetermined information exchange). In addition, device restrictions according to antenna design and deployment influence application of MIMO technology. As another problem, network types and bandwidths supported by the transmission device and the reception device match. That is, when the transmission device and the reception device support one or more communication systems, the network types and operating bandwidths currently applied to both devices must match. This problem may be internally solved when identical systems are used in both devices. However, if different systems are used in both devices, a separate network detection scheme needs to be applied or information about a network must be directly input by a user. Such problems will be described in detail with reference to FIG. 1.
FIG. 1 shows an example in which two devices supporting various communication systems communicate with each other. In FIG. 1, A, B, C and D indicate Wi-Fi, Bluetooth, Global System for Mobile Communications (GSM), LTE communication systems, respectively. In the example of FIG. 1, a source device supports all four communication systems, but a destination device supports only Wi-Fi, GSM and LTE communication systems. In the case of communication using Wi-Fi and GSM, the source device performs transmission/reception using a single antenna and the destination device performs transmission and reception using two antennas. If a multi-band antenna is applied due to limited device size, GSM and LTE may share antennas. That is, the source device may use one of the two antennas for LTE as an antenna for GSM and the destination device may share two antennas for LTE and GSM. In case of LTE, the source device and the destination device perform transmission/reception using both antennas. In case of Bluetooth, since only the source device has a Bluetooth module, communication between the source device and the destination device through Bluetooth is impossible.
Referring to FIG. 1(b−1) which shows direct communication between the source device and the destination device in a MIMO scheme, in direct communication between the devices using Wi-Fi, since the source device has only one antenna, only a receive diversity scheme of the destination device may be considered (if a relationship between the source device and the destination device is changed, only a transmit diversity scheme may be considered). Although direct transmission between devices in a cellular system, such as GSM or LTE, is not currently supported, a radio frequency (RF) signal amplifier may be considered. In this case, since a method of sharing information regarding antennas and networks between devices is not currently supported, access is possible in a method similar to Wi-Fi. In the example of FIG. 1(b−1), in case of GSM, since the destination device uses one antenna for GSM and the source device uses two antennas for GSM, only a receive diversity scheme of the destination device may be considered. In addition, in case of LTE, since each of the destination device and the source device has two antennas for LTE, a transmit/receive diversity scheme may be used. In case of cellular communication, only a scheme capable of performing transmission/reception even in a state in which a device does not know a transmission scheme/antenna configuration of a counterpart device may be used.
In FIG. 1, from the viewpoint of operating bandwidth, since a cellular system does not support control information exchange between devices yet, the device may not know information regarding operating bandwidth. Accordingly, the system needs to perform signal processing with respect to operating bandwidth.
MIMO systems must be deployed to meet demands for data transfer rate scheme. However, as in a cellular communication example of FIG. 1, if per-system antenna information and network information of each device is not known, an available MIMO scheme is very restrictive. Further, if multiple antennas are used, it is difficult to perform transmission of multiple streams and to solve interference between antennas.
For example, as shown in FIG. 2, if each of a source device and a destination device has two antennas and the source device transmits two streams (s1 and s2), it is difficult for the destination device to distinguish between the two streams due to interference (s2h21 and s1h12). In addition, it is also difficult to support rank adaptation for adaptively adjusting the number of streams transmitted between devices.
In particular, if near-field communication is considered in order to reduce long-term fading, interference between antennas shown in FIG. 2 may become severe, because performance sensitivity is significantly increased due to beam mismatch and antenna interference between two devices as a distance between the devices decreases.
FIG. 3 shows beam regions and interference influence between respective antennas of two devices and/or between antennas of each device, which may occur when the number of antennas, the locations of the antennas, etc. are not known in short-range communication between a source device and a destination device.
FIG. 3(a) shows beam regions and interference if a source device and a destination device have a similar antenna location and type/kind. As shown in FIG. 3(a), antennas #1 and #2 of the source device and the destination device have a high signal to interference plus noise ratio (SINR) because beam patterns thereof match.
FIG. 3(b) shows beam regions and interference if a source device and a destination device are different in at least one of antenna location and type/kind. In this case, antennas #1 and #2 of the source device and the destination device have an SINR relatively lower than that of FIG. 3(a) because beam patterns thereof do not match each other.
Such performance sensitivity may cause more severe problems if a terminal has multiple antennas. This is because mutual interference between antennas occurs or an SINR between antennas is changed according to relative position between the antennas of the source device and the antennas of the destination device. For example, assume that the antenna #1 of the source device and the antenna #1 of the destination device match but the antenna #2 of the source device and the antenna #2 of the destination device do not match. In this case, in downlink, a difference in channel gain (or path loss) between receive antennas of the source device is increased. Thus, it is difficult to perform transmission of high rank which is expected due to use of multiple receive antennas, that is, simultaneous transmission of multiple streams (or layers). In addition, space multiplexing effect, that is, reception stability increase obtained by combining signals received by a plurality of receive antennas having different channel properties is also reduced. Similarly, in uplink, since channel gain varies according to channel gain between transmit antennas of the source device, a probability that transmission of high rank is performed is reduced and transmit diversity is also reduced.